Subscriber line interface circuits are typically found in a central office exchange of a telecommunications network. A SLIC provides a communications interface between the digital switching network of the central office and an analog subscriber line. The analog subscriber line connects to a subscriber station or telephone instrument at a location that is remote from the central office exchange.
The analog subscriber line and subscriber equipment (e.g., a telephone) form a subscriber loop. The interface requirements of a SLIC typically cause high voltages and currents for control signaling with respect to the subscriber equipment on the subscriber loop. Voiceband communications are typically low voltage analog signals on the subscriber loop. Accordingly, the SLIC performs various functions with respect to voiceband and control signaling between the subscriber equipment and the central exchange.
Another function a SLIC typically performs is pulse metering. Pulse metering is used to provide a pulse sequence that is indicative of a cost or other billing information of an ongoing communication (e.g., a billing tone). For example, a series of metering pulses can be generated by a SLIC. The number of these pulses can be indicative of a cost of an ongoing communication, and may further be used to disable a communication device, such as a telephone, after a given amount of pulses have occurred (e.g., after phone charges for a call meets the amount paid for the call). Meter pulses are generally generated at a frequency outside of the audio band, typically at 12 kHz or 16 kHz.
Circuitry within a SLIC can be shared to perform both pulse metering and audio transmission. However, the generation of meter pulses can create noise, which can undesirably affect audio quality of a phone call or other communication. Noise can also be an issue at much higher frequencies, such as those at which a digital subscriber line (DSL) modem coupled to a telephone line operates. Specifically, harmonics of noise in meter pulses can cause noise at these higher frequencies, which typically range from 25 kHz to 2,200 kHz.
Another problem with sharing circuitry between an audio path and a pulse metering path is that pulse metering operates on a different impedance model than the audio path. That is, SLICs typically connect to a telephone line. Such telephone lines are made of low quality copper wire. Accordingly, phone lines are typically modeled with a relatively high impedance for audio transmission (e.g., 600 ohms (Ω)). In contrast, the pulse metering path through the phone lines is typically modeled with a much lower impedance (e.g., 200Ω). Thus pulse metering operates on a lower impedance, R, and thus requires more current, I, to achieve a certain voltage (V=IR). In contrast, audio operates on a higher impedance and thus needs less current. In the example described herein, the audio requires only one third of the current gain because it has a three times larger impedance level.
Another design consideration is that analog oscillators used in generating metering pulses consume significant area. Accordingly, some SLICs use a digital oscillator to form a digitally generated sine wave. However, such a digitally generated sine wave can itself cause undesired noise, specifically, quantization noise. To reduce such noise, digital-to-analog converters (DAC's) having a relatively larger number of bits are used. Thus, DACs used in pulse generation circuitry typically have a high resolution, providing greater dynamic range to reduce the effects of noise. However, such higher resolution DACs are more expensive and consume greater chip real estate.
Because noise is a concern in the audio band, circuitry used to generate both pulse metering signals and audio signals needs to have sufficient resolution to reduce the noise to acceptable levels. That is, the more bits used to represent a sample will reduce the quantization noise, increasing the signal to noise ratio (SNR), improving resolution. However, such circuitry is expensive and consumes significant chip real estate.
Additionally, the problem of using a digital oscillator is compounded by the fact that audio and pulse metering paths work on different impedances and therefore require different gains. To resolve this issue of gain differences, DACs having more bits are used, increasing area. If a single DAC is used for both pulse metering and audio functions, three times the current gain is used, even though it is not needed for the audio portion. Because this gain increases the noise, the DAC must have a sufficiently large resolution to overcome the noise issue.
In other words, noise in a SLIC is constrained by an audio specification which requires low gain. However, the gain is constrained by a pulse metering specification, which requires a large gain value. These constraints often lead to use of expensive high resolution DACs in conventional SLICs.
Accordingly, a need exists to provide pulse metering functions while reducing noise at audio levels and at DSL levels.